A method and a device are described in International Published Patent Application No. WO 00/05696, in which the safety procedure implemented between the electronic key and the base station is executed in such a way that the communication between the electronic key and the base station is performed in the active operating mode via UHF-frequencies, the range of the transmission between the electronic key and the base station being limited in order to ensure that the communication link is interrupted when the person holding the key moves out of the immediate vicinity of the secured location, for example, the motor vehicle.
In order to prevent such a passive access control system from being inactivated by an unauthorized attacker intercepting the call signal transmitted by the base station to the electronic key, forwarding the intercepted signal via a radio link extension to a second attacker who is in the vicinity of the electronic key, and the second attacker retransmitting to the first attacker via the radio link extension the reply signal of the electronic key in response to the call signal of the base station and sending it via this attacker back to the base station, in the conventional method, the electronic key transmits a signal to the base station, which is then converted by the base station into spectral data. The base station will grant access to the secured location only if these spectral data in the transmission of the authentication data match a spectral signature of the electronic key that is stored in the base station. In this context, the signal transmitted by the electronic key includes at least two tones of different frequencies f1 and f2, respectively, and the spectral data represent tones of the third order of the transmitted signal, which are measured by the base station at the frequencies 2f1−f2 and 2f2−f1. If the received signal strength of these secondary lines of the signal transmitted by the electronic key exceeds a predefined value, the base station interprets this as a reliable indication that the radio link has been extended, and refuses access to the secured location.
In order to still allow the user to enter the secured location when the active communication mode is inoperative, within the framework of a so-called back-up mode, i.e., a passive communication mode, it is provided that in this passive communication mode, a data transmission between the electronic key and the base station be performed by a passive modulation of the exciter field transmitted by the base station. The electronic key detunes its resonance circuit in correspondence with the data to be transmitted, which may be detected by the base station as an additional load on its resonant circuit. This passive communication conducted on LF-frequencies is limited to a few centimeters, which means that a potential attacker, when attempting to send the data signals transmitted by the key to the base station on an LF-frequency in back-up mode, must place his respective antenna very close to the transmitting antenna of the base station in order to operate in this back-up mode. The conventional methods and the conventional devices working according to these methods have the disadvantage of not providing effective protection against an attack performed in the aforementioned manner.
Therefore, it is an object of the present invention to provide a method and a device that provides effective protection against a radio link extension in the passive communication mode.